ENOSTOSIS

Typically appears as a small, well-defined, radiodense lesion located within the trabecular bone, often in the proximal femur or pelvis. It is important to differentiate it from more aggressive lesions, and its appearance is best seen on CT, which shows the lesion’s endosteal location and the preservation of cortical integrity (17, 18).

Fig 1: MRI shows well-defined sclerotic lesion in the right femoral head-neck transition.

OSTEOID OSTEOMA 

Usually presenting in the long bones, is characterized by a small, central nidus surrounded by a sclerotic zone, best visualized on CT scans. On MRI, the nidus may show up as a focal area of low signal intensity, while the surrounding sclerosis appears as a high-signal zone (1).

Fig 2: (A, B) Axial CT show central nidus surrounded by a sclerotic zone. (C, D) CT reconstruction and radiography show diffuse thickening of the cortex, with small central nidus.

SUBCHONDRAL CYSTS 

Offten associated with degenerative joint diseases and present as fluid-filled cavities beneath the articular cartilage, usually seen on plain X-rays or MRI. Their well-demarcated, radiolucent appearance can help distinguish them from more aggressive lesions like bone tumors (5).

Fig 3: (A) Coronal CT shows big well-defined fluid-filled lesion in the hip joint. (B, C) Coronal MRI (two sequences) evidence fluid-filled lesion near to the articular surface, hyperintense on T2-weighted sequence, consistent with subchondral cyst.

FIBROUS DYSPLASIA 

This condition involves replacement of normal bone with fibrous tissue, demonstrates a characteristic "ground-glass" appearance on both X-rays and CT scans. MRI may show heterogeneous signal intensity, reflecting the fibrous and osseous components (1).

Fig 4: (A, B) X-ray demonstrates ground-glass opacities, cystic well-defined borders and expansion of the bone, with intact overlying bone.

Fig 5: Frontal X-ray of the face shows marked bone expansion and increased radiopacity, particularly affecting the maxilla and, mandible. There is significant facial asymmetry, with thickening and deformation of the involved structures. Cherubism.

Fig 6: Fibrous dysplasia. Coronal CT of the face reveals an extensive, heterogeneous, and expansile bone lesion, thickened and irregular, with a "ground-glass" appearance. There is significant asymmetry, with involvement of the frontal, sphenoid, and maxillary bones. The orbit and adjacent structures appear to be affected.

Fig 7: Axial CT of the face shows ground-glass opacity in the left maxillary sinus. Fibrous Dysplasia.

CHONDROMYXOID FIBROMA

It is a rare benign cartilaginous tumor that typically appears as a lobulated, well-defined, radiolucent lesion on X-ray, often in the metaphysis of long bones. On MRI, it shows mixed low and intermediate signal intensity with peripheral enhancement after contrast. A multimodal imaging approach is crucial for accurate diagnosis, allowing for detailed evaluation of the lesion's size, location, and characteristics, helping to distinguish it from more aggressive bone pathologies (2).

Fig 8: X-ray shows an eccentric bubbly and lucent lesion in the distal tibial metaphysis with sclerotic margins.

NON-OSSIFYING FIBROMA (NOF) 

Typically found in the metaphysis of long bones in children and adolescents, is characterized by a well-defined, eccentric, radiolucent lesion with a thin sclerotic rim on X-ray. CT scans demonstrate a cystic lesion with a trabecular bone appearance and minimal to no periosteal reaction, which helps distinguish it from malignant lesions (1, 2).

Fig 9: X-ray demonstrates a lytic lesion with lobulated contour and sclerotic margins, eccentrically located in the distal femur metaphysis, with focal thinning of the adjacent cortex.

OSSIFYING FIBROMA 

It is more common in the craniofacial bones, presents as a well-circumscribed, expansile lesion with varying degrees of ossification. On CT, it demonstrates a mixed-density pattern with both radiolucent and radiopaque areas, and on MRI, it shows low-to-intermediate signal intensity with peripheral enhancement (10).

Fig 10: Coronal CT shows a expansile, well-defined, heterogeneous lesion in the jaw. There is evidence of intracortical osteolysis with a characteristic sclerotic band (osteoblastic rimming).

CORTICAL FIBROUS DEFECTS

These lesions are usually asymptomatic and present as small, oval or round, radiolucent lesions with a sclerotic border, often found in the distal femur or proximal tibia. These defects are typically smaller than NOF and lack the expansile nature of ossifying fibroma, making them easily distinguishable on imaging (11, 12).

Fig 11: (A, B, C) Lucent tibial diaphysial intracortical defect outlined by a thick rim of sclerosis with no clear involvement of the tibial medullary cavity and with no periosteal reaction.

SIMPLE BONE CYSTS (SBC) 

SBC are fluid-filled lesions that appear as well-circumscribed, radiolucent areas in the metaphysis of long bones, commonly in the proximal humerus or femur. On X-ray, they may demonstrate a "fallen fragment sign", where a fragment of bone is displaced within the cystic space. CT and MRI provide further clarity, with MRI showing a well-defined cyst with low signal intensity on T1 WI and high signal intensity on T2 WI (1, 2).

Fig 12: X-ray evidences well circumscribed, expansile, lucent lesion in the humerus metaphysis. In the middle of the lesion, a pathological fracture is noticed, coursing transversely through the bone.

ANEURYSMAL BONE CYSTS (ABCs) 

Expansile lesions with a characteristic multilocular appearance and a "bubbly" or "soap-bubble-like" pattern visible on X-ray or CT. On MRI, ABCs are hyperintense on T2-weighted images, with internal septations and peripheral rim enhancement after contrast administration, which helps differentiate them from other cystic lesions (1, 2).

Fig 13: X-rays demonstrate an expansile, lucency in the upper tibial metaphysis. The lesion does not transgress the growth plate.

BONE INFARCTS 

Bone infarcts are typically seen in the femoral head, are often the result of ischemia and present with a subchondral lucency and crescent sign on X-ray, indicating collapse of the bone structure. CT and MRI are more sensitive, revealing areas of necrosis with an irregular border and low signal intensity on T1-WI and high signal intensity on T2 WI (2).

Fig 14: Coronal MRI (two sequences) show bone lesion with a geographic aspect and double contour in the distal metadiaphyseal region of the femur.

ENCHONDROMAS 

Benign cartilage lesions which typically appear as radiolucent areas with calcified, ring-like or stippled patterns within the lesion on X-ray. CT reveals a lobulated mass with areas of calcification, while MRI shows intermediate signal intensity on T1 and high signal intensity on T2, reflecting the cartilaginous content (1-3).

Fig 15: 1 (A, B) MRI in T1- and T2-weighted sequences shows a well defined lobulated intramedullary metadiaphyseal lesion in the distal femur.

Fig 16: 2 (A, B) X-rays demonstrate a well defined lytic lesion at the fifth metacarpal (C) and fourth proximal phalanx (D).

CALCIFYING CHONDROMAS

Rare benign cartilage tumors that present as well-defined, radiolucent lesions with calcification in the central portion, visible on X-ray. On CT, they demonstrate a mixture of calcified and non-calcified areas, and MRI shows a heterogeneous signal intensity, with regions of low signal intensity due to calcifications (1-3).

Fig 17: (A, B) X-rays images show well-defined, lobulated radiolucent areas with central stippled or popcorn-like calcifications, which are characteristic of enchondromas. Courtesy of Dr. Armando Abreu - Porto Alegre, Brazil.

OSTEOCHONDROMAS 

One of the most common benign bone tumors, appear as bony exostoses with a cartilage cap, seen on X-ray as a sessile or pedunculated growth extending from the cortical surface. MRI is particularly useful in assessing the thickness of the cartilage cap, as an abnormal increase in thickness may suggest malignant transformation (2).

Fig 18: (A) Coronal MRI with bilateral bony projections in the femur, away from the epiphysis. (B) X-ray evidences pedunculated projection away from the epiphysis.

MULTIPLE HEREDITARY EXOSTOSES (MHE) 

MHE is an autosomal dominant condition that  presents with multiple osteochondromas scattered throughout the skeleton, most often in the metaphyses of long bones. These lesions appear as multiple bony protrusions on X-ray, with cartilage caps visible on MRI. The pattern of lesion distribution and the presence of a cartilage cap are key to diagnosing MHE and distinguishing it from solitary osteochondromas (2, 16).

Fig 19: (A, B) MRI evidences protrusions in the inferomedial border of the scapula in T1- and T2-weighted sequences, with a thin cartilaginous cap. (C, D) CT shows bony protusions in the scapula and metaphysis of femur.

MELORHEOSTOSIS

Melorheostosis is a rare sclerosing bone disorder, typically seen on radiographs with a "flowing" or "dripping candle-wax" appearance, where cortical sclerosis extends along the bone shaft. X-rays show irregular cortical thickening with a serpentine pattern. CT provides a more detailed view of the sclerosis, and MRI usually shows low signal intensity across all sequences due to the dense cortical sclerosis (6-8).

Fig 20: X-rays show periosteal cortical thickening, with the "dripping candle-wax" appearance, in the tibia.

CAFFEY SYNDROME

This is a rare disorder in infants, that is characterized by periosteal reaction and cortical thickening, particularly in the long bones, with swelling of soft tissues. Radiographs show periosteal new bone formation along the shafts of long bones, with cortical thickening and the characteristic "onion-skin" appearance. MRI reveals soft tissue swelling with abnormal signal intensities corresponding to the periosteal reactions. Though self-limiting, it can cause significant skeletal deformities in some cases (9, 10).

Fig 21: A) X-ray shows finding of periostitis along the left ribs in a infant with left chest wall edema. (B) X-ray evidences visible jaw swelling, indicating periostitis, in a Infant with fever, irritability and facial edema. Images (A, B) are courtesy of Dr. Silvio Cavalcanti and Dr. Eduardo Just – Recife, Brazil

MAFFUCCI SYNDROME

Maffucci syndrome is characterizaed by multiple enchondromas and soft tissue hemangiomas. On radiographs, the enchondromas appear as well-defined, radiolucent lesions with calcified ring-like patterns. MRI is helpful for detecting hemangiomas, which appear as well-defined, hyperintense lesions on T2-WI. This syndrome also has a higher risk of malignant transformation of enchondromas, making imaging surveillance critical (4).

Fig 22: (A) x-ray shows multiple bilateral enchondromas in the in the hand, associated with multiple subcutaneous tumors, some with calcifications, with asymmetrical distribution, suggestive of large hemangiomas. (B) x-ray of the hips evidences multiple enchondromas, in the ilium, ischiopubic ramus and femur bilaterally.

OLLIER DISEASE

Ollier disease is similar to Maffucci syndrome but lacks soft tissue hemangiomas. The hallmark on radiographs is multiple well-defined enchondromas, most commonly in the metaphyses of long bones. CT and MRI may reveal lobulated masses with calcified areas. MRI is especially useful for monitoring changes in bone marrow and detecting malignant transformation, which can occur in some cases (3, 4).

Fig 23: X-ray evidences many of the bones of the hand showing lucent expansion with sharp zone of transition and non-aggressive appearance. Some have central calcification.

ENGELMANN-CAMURATI SYNDROME (ECS)

ECS, known as progressive diaphyseal dysplasia, is an uncommon disease. The diagnosis is based on the clinical symptoms and laboratory findings, but is confirmed by the radiological images. The characteristic radiological manifestations are fusiform cortical thickening of the long bones, bilateral and symmetric. CT and MRI help assess bone involvement, with MRI showing low signal intensity due to sclerosis and can also evaluate associated muscle atrophy (15).

Fig 24: (A, B) There is diffuse and irregular thickening of the cortical bone in the diaphysis of tibia and fibula, with no involvement of the epiphysis.

PAGET’S DISEASE

Paget disease is a chronic bone disorder characterized by excessive bone remodeling, usually affecting the pelvis, spine and long bones. Radiographs initially show a "coarsened" trabecular pattern, with cortical thickening and enlargement of affected bones, and a mixed lytic and sclerotic pattern. As the disease progresses, bones may become more brittle and deformed, with a characteristic "cotton-wool" appearance in the skull. CT and MRI provide better visualization of trabecular changes and cortical thickening, with MRI showing variable signal intensity depending on the phase of the disease (13, 14).

Fig 25: (A, C) evidence both sclerotic and lytic lesions, in association with loss of distinction between cortices and medullary cavity. (B) X-ray shows “cotton wool” appearance, ill-defined sclerotic and lucent areas throughout. The cortex is thickened and irregular.